1. Field of the Invention
The present invention relates to a lightened disk for a wheel and a method for manufacturing the same.
2. Description of Related Art
A conventional method for manufacturing a conventional disk for an automobile wheel is illustrated in FIG. 8. As illustrated in FIG. 8, a flat blank 12 is stamped from a flat plate 11. The blank 12 is then plastically deformed into a disk 13 by pressing the blank at least one time, usually four or five times. Due to an increase or decrease in a thickness during press-forming, portions having a thickness greater than a thickness of the flat blank or having; a thickness smaller than a thickness of the flat blank are formed in the disk 13. More particularly, as illustrated in FIG. 9, a curved portion B having a small radius is thinner than the flat blank, and a portion A reduced in diameter (the flange portion 13a) is thicker than the flat blank. Measuring the disks having a relatively long flange portion, it was found that a thickness at the axially inboard edge of the flange portion increases to a thickness 1.5 times greater than the thickness of the disk material (flat blank). When the blank is deformed to the flange portion accompanied by a decrease in diameter, since the cross section does not change, the thickness of the flange portion is increased in proportion to a ratio D1/D2, where D1 is a diameter before deforming and D2 is a diameter after deforming. Further, the flange portion includes an axially inboard edge 14 which includes a general portion 14a corresponding to an arc portion 12a of the flat blank 12 and a curved portion (ventilation portion) 14b corresponding to a straight side 12b of the flat blank 12, and the thickness increase at the ventilation portion 14b, especially the center of the ventilation portion, is greater than other portions. This is caused because the axial length of the ventilation portion is smaller than the general portion and a resistance against the diameter reduction is smaller at the ventilation portion than at the general portion, so that material around the ventilation portion flows into the ventilation portion during plastic deforming and the great thickness increase occurs. This thickness increase has been thought to be an inevitable consequence of plastic deformation.
However, there are the following problems with the conventional disk for a wheel.
A stress and stress variation generated in the flange portion of the disk is much smaller than that generated in other portions of the wheel. There is substantially no breakage at the flange portion. Therefore, there will be no problem from the standpoint of strength even if the thickness of the flange portion is reduced. Irrespective of that, since the flange portion of the conventional disk is thicker than other portions of the disk due to thickening during deforming, increase in weight and cost occurs.